Symmetric blocking transient voltage suppressor (TVS) using bipolar transistor base snatch

ABSTRACT

A symmetrical blocking transient voltage suppressing (TVS) circuit for suppressing a transient voltage includes an NPN transistor having a base electrically connected to a common source of two transistors whereby the base is tied to a terminal of a low potential in either a positive or a negative voltage transient. The two transistors are two substantially identical transistors for carrying out a substantially symmetrical bi-directional clamping a transient voltage. These two transistors further include a first and second MOSFET transistors having an electrically interconnected source. The first MOSFET transistor further includes a drain connected to a high potential terminal and a gate connected to the terminal of a low potential and the second MOSFET transistor further includes a drain connected to the terminal of a low potential terminal and a gate connected to the high potential terminal.

This Patent Application is a Continuation Application and claim thePriority Date of application Ser. No. 12/456,948 filed on Jun. 25, 2009.The application Ser. No. 12/456,948 is a Divisional Application andclaims the Priority Date of another application Ser. No. 11/541,370, nowissued as U.S. Pat. No. 7,554,839, filed on Sep. 30, 2006 by commonInventors of this Application. The Disclosures made in the PatentApplication and application Ser. Nos. 12/456,948 and 11/541,370 arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a circuit configuration and method ofmanufacturing a transient voltage suppressor (TVS). More particularly,this invention relates to an improved circuit configuration and methodof manufacturing a symmetrical bi-directional blocking transient voltagesuppressor (TVS) implemented with bipolar transistor base snatch toconnect the base to a low potential terminal.

2. Description of the Relevant Art

The conventional technologies for designing and manufacturing abi-directional blocking transient voltage suppressor (TVS) is stillconfronted with a technical difficulty that the base of a TVS device isconnected to a terminal of floating potential. Typically, thebi-directional blocking TVS is implemented with symmetric NPN/PNPconfiguration with identical emitter-base and collector-base breakdownvoltage. However, such implementation often leads to a floating basethat further leads to difficulties of voltage variations over time,i.e., the dV/dt. The voltage variation over time further leads to theleakage current concerns due to the facts that when the base isfloating, the voltage change dV/dt causes the equivalent capacitorgenerating charging and discharging currents that increase the leakagecurrent.

The transient voltage suppressors (TVS) are commonly applied forprotecting integrated circuits from damages due to the inadvertentoccurrence of an over voltage imposed onto the integrated circuit. Anintegrated circuit is designed to operate over a normal range ofvoltages. However, in situations such as electrostatic discharge (ESD),electrical fast transients and lightning, an unexpected and anuncontrollable high voltage may accidentally strike onto the circuit.The TVS devices are required to serve the protection functions tocircumvent the damages that are likely to occur to the integratedcircuits when such over voltage conditions occur. As increasing numberof devices are implemented with the integrated circuits that arevulnerable to over voltage damages, demands for TVS protection are alsoincreased. Exemplary applications of TVS can be found in the USB powerand data line protection, Digital video interface, high speed Ethernet,Notebook computers, monitors and flat panel displays.

FIGS. 1A and 1B show a circuit diagram and a current-voltage diagramrespectively of a TVS device. An idea TVS is to totally block thecurrent, i.e., zero current, when the input voltage Vin is less than thebreakdown voltage VBD for minimizing the leakage current. And, ideally,the TVS has close to zero resistance under the circumstance when theinput voltage Vin is greater than the breakdown voltage VBD such thatthe transient voltage can be effectively clamped. A TVS can beimplemented with the PN junction device that has a breakdown voltage toallow current conduction when a transient input voltage exceeds thebreakdown voltage to achieve the transient voltage protection. However,the PN junction type of TVS has no minority carriers and has a poorclamping performance as that shown in FIG. 1B. There are alternate TVSimplementations with Bipolar NPN/PNP with an Avalanche triggeredturning-on of the Bipolar transistor. The base is flooded with minoritycarriers and the bipolar TVS can achieve better clamping voltage.However, the avalanche current is amplified with the bipolar gain.

With the advancement of electronic technologies, there are increasinglymore devices and applications that require bi-directional TVSprotections. Devices for audio, ADSL, multiple-mode transceivers, andother electronic devices are required to provide the bi-directional TVSprotections as these electronic devices are manufactured with componentsmore vulnerable to transient voltages and operated under more differentkinds of conditions that the transient voltage may occur either aspositive or negative transient voltages. Currently, the most effectivetechnique to provide the bi-directional TVS is to implement a symmetricNPN/PNP configuration with identical Emitter-Base and Collector-Basebreakdown voltage. However, as discussed above, in a conventionalbi-directional TVS device as that shown in FIGS. 2A and 2B, to haveeither symmetrical clamping or unsymmetrical clamping respectively, thebase of the TVS in these NPN/PNP circuits are left at a floatingpotential in order to achieve symmetrical breakdown. The floating basein these implementations causes the dV/dt issues and also the leakageconcerns.

Therefore, a need still exists in the fields of circuit design anddevice manufactures for providing a new and improved circuitconfiguration and manufacturing method to resolve the above-discusseddifficulties. Specifically, a need still exists to provide new andimproved TVS circuits that can provide bi-directional symmetricalblocking of transient current to achieve TVS protection by implementingNPN/PNP transistors where the base is always connected to the terminalwith a potential such that the above discussed problems and difficultiesare resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide abi-directional symmetrical blocking TVS with a base that is connected toa lower potential such that the above-discussed difficulties andlimitations encountered by the conventional bi-directional blocking TVScaused by a floating base can be overcome.

Another aspect of the present invention to provide a bi-directionalsymmetrical blocking TVS with a base connected to a lower potential andthe TVS is implemented with either lateral or vertical configurations byapplying the integrated circuit (IC) manufacturing processes.

Briefly in a preferred embodiment this invention discloses a symmetricalblocking transient voltage suppressing (TVS) circuit for suppressing atransient voltage. The symmetrical blocking transient voltagesuppressing (TVS) circuit includes a bipolar transistor having a baseelectrically connected to a common source of two MOS transistors wherebythe base of bipolar is tied to an emitter potential of the bipolartransistor in either a positive or a negative voltage transient. Inanother preferred embodiment, the two MOS transistors are twosubstantially identical transistors for carrying out a substantiallysymmetrical bi-directional clamping a transient voltage. These two MOStransistors further include a first and second MOSFET transistors havingan electrically interconnected source. The first MOSFET transistorfurther includes a drain connected to a high potential terminal and agate connected to the terminal of a low potential and the second MOSFETtransistor further includes a drain connected to the terminal of a lowpotential terminal and a gate connected to the high potential terminal.In one embodiment, the symmetrical blocking transient voltagesuppressing (TVS) circuit includes a NPN bipolar transistor having abase electrically connected to a common source of two MOS transistors, acollector connected to the high potential terminal and an emitterconnected to the terminal of a low potential. In another embodiment, thesymmetrical blocking transient voltage suppressing (TVS) circuitincludes a PNP bipolar transistor having a base electrically connectedto a common source of two MOS transistors, a collector connected to thelow potential terminal and an emitter connected to the terminal of ahigh potential.

In another embodiment the first MOSFET transistor and the second MOSFETtransistor further include two lateral MOSFET transistors extendedlaterally along a first direction of a semiconductor substrate anddisposed laterally on two opposite sides of a doped region functioningas a base of the NPN transistor extending along a second direction overthe semiconductor substrate perpendicular to the first direction. Thefirst and second MOSFET transistors are encompassed in two N-wellregions disposed laterally on two opposite sides of the doped regionfunctioning as the base of the NPN bipolar transistor wherein the twoN-well regions functioning as an emitter and a collector of the NPNtransistor. The first MOSFET transistor and the second MOSFET transistorand the NPN bipolar transistor are manufactured by applying a CMOSmanufacturing process.

In another preferred embodiment, the present invention further disclosesan electronic device formed as an integrated circuit (IC) wherein theelectronic device further includes a symmetrical blocking transientvoltage suppressing (TVS) circuit. The first MOSFET transistor and thesecond MOSFET transistor of the TVS circuit further includes two lateralMOSFET transistors sharing a common source region encompassed in aP-body region functioning as the base of the NPN transistor. The NPNtransistor further includes a vertical NPN transistor with the commonsource region functioning as a cathode terminal disposed above theP-body region functioning as the base region and a doped substrate layerdisposed below the P-body region as an anode terminal of the NPNtransistor. The first and second MOSFET transistors further include twolateral MOSFET transistors and the NPN transistor further includes avertical NPN transistor manufactured by applying a DMOS manufacturingprocess.

The present invention further discloses a method for manufacturing anelectronic device with an integrated symmetrical blocking transientvoltage suppressing (TVS) circuit. The method includes a step ofelectrically connecting a base of an NPN transistor to a common sourceof two transistors to tie the base to a terminal of a low potential ineither a positive or a negative voltage transient. The method furtherincludes a step of manufacturing the two transistors as twosubstantially identical transistors for carrying out a substantiallysymmetrical bi-directional clamping a transient voltage. In a preferredembodiment, the method further includes a step of manufacturing the twotransistors as a first and second MOSFET transistors having anelectrically interconnected source for electrically connecting to thebase of the NPN transistor. In a preferred embodiment, the methodfurther includes a step of connecting a drain of the first MOSFETtransistor to a high potential terminal and connecting a gate of thefirst transistor to the terminal of a low potential. The method furtherincludes connecting a drain of the second MOSFET transistor to theterminal of a low potential terminal and connecting a gate of the secondMOSFET transistor to the high potential terminal. In another preferredembodiment, the method further includes a step of extending laterallythe first MOSFET transistor and the second MOSFET transistor along afirst direction of a semiconductor substrate on two opposite sides of adoped region; and extending the doped region along a second directionover the semiconductor substrate perpendicular to the first directionfor functioning as a base of the NPN transistor. In another embodiment,the method further includes a step of encompassing the first and secondMOSFET transistors in two N-well regions disposed laterally on twoopposite sides of the doped region as the base of the NPN transistor;whereby the two N-wells functioning as an anode and a cathode for theNPN transistor. In an exemplary embodiment, the method further includesa step of applying a CMOS manufacturing process to manufacture the firstand second MOSFET transistors and the NPN transistor.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram for showing a conventional TVS device andFIG. 1B is an I-V diagram, i.e., a current versus voltage diagram, forillustrating the reverse characteristics of the TVS device.

FIG. 2A shows a circuit diagram along with waveforms for illustratingthe functions performed by the unidirectional device of asymmetricclamping of a unidirectional TVS.

FIG. 2B shows a circuit diagram along with waveforms for illustratingthe functions performed by the bi-directional device of symmetricclamping of a bi-directional TVS.

FIGS. 3A and 3B are circuit diagrams of bi-directional symmetricalclamping TVS of this invention with the base always connected to thebipolar emitter potential.

FIG. 4 is a perspective view for showing a lateral implementation of theTVS of FIG. 3A.

FIG. 5 is a cross sectional view for showing a vertical implementationof the TVS of FIG. 3A.

FIGS. 6A to 6C are perspective views for showing standard CMOS processesfor manufacturing a symmetrical TVS device of FIG. 4.

DETAILED DESCRIPTION OF THE METHOD

Referring to FIG. 3A for a circuit diagram of new and improved TVS thathas a bi-directional symmetric blocking function. The TVS 100 includestwo MOSFET transistors. The first transistor 110 shown as M1 has asource terminal 110-S connected to a source terminal 120-S of a secondtransistor 120 shown as M2. The first transistor 110 further has a drainterminal 110-D connected to a high voltage terminal 105 with a voltageVcc and a gate terminal 110-G connected to a ground terminal 99 with avoltage GND. The second transistor 120 further has a drain terminalconnected to the ground terminal GND and a gate terminal connected tothe high voltage terminal Vcc. The interconnected point 125 of thesource terminals of the first transistor 110 and second transistor 120is further connected via a resistor 130 to a base of a NPN transistor140 connected in parallel between the main voltage terminal Vcc and theground terminal GND, with emitter connected to GND and collectorconnected to Vcc.

During a positive ESD event when Vcc>GND, the second transistor 120 isturned on while the first transistor 110 is turned off and the base ofNPN transistor 140 is grounded through the resistor 130. During anegative ESD event when Vcc<GND, the second transistor 120 is turned offwhile the first transistor 110 is turned on and the NPN transistor 140is connected to the main voltage terminal Vcc through the resistor 130.In either case the NPN base is connected to the terminal with lowerpotential. The PN junction breaks down when the transient voltage exceedthe designated breakdown voltage thus clamp the voltage at thedesignated level. A symmetrical bi-directional block is achieved. Unlikethe floating base as that implemented in a conventional TVS, the base isconnected to a terminal lower potential through the resistor 130 andgreatly reduce the charging and discharging current.

FIG. 3B is a circuit diagrams of bi-directional symmetrical clamping TVSof this invention implemented with PNP bipolar transistor. The TVS 100′is similar to TVS 100 of FIG. 3A except that the PNP has an emitterconnected to Vcc and a collector connected to GND. The operationprinciple is the same as FIG. 3A.

The bi-directional symmetrical-blocking TVS as shown in FIG. 3A isimplemented as a lateral device shown in FIG. 4 in a P Epi layer 155formed on top of a P+ substrate 150. There are two N-wells 140-C and140-E formed laterally around a P-well 140-B provided by Epi layer 155to function as a lateral NPN transistor 140. Part of P-well extendedfrom a body contact P+ region next to the source region, i.e., terminal125 to provide the distributive resistor Rs 130. The resistor 130 isconnected to the source terminals 110-S and 120-S of the first andsecond MOSFET transistors 110 and 120 disposed laterally on two oppositesides of the P-well 130. The collector 140-C of the NPN transistor 140,the gate 110-G and the drain 120-D are connected to the cathode or GNDterminal while the emitter 140-E of the NPN transistor 140, the gate120-G and the drain 110-D are connected to the anode or Vcc through atwo-metal-layer contact scheme (not shown). A symmetrical Bi-directionalblocking TVS can be conveniently manufactured by applying standard CMOSmanufacturing processes.

FIG. 5 shows another implementation where the NPN transistor 140 isformed vertically with a N+ region 120-D disposed on top of a P-well140-B over a bottom N+ substrate 150′r that connected to the groundterminal 99. P-well 140-B also functions as the body of the MOStransistors. A P+ region 125 is placed next to a source regions 110-Sand 120-S for the two MOSFET devices 110 and 120 for source/body shortcontact. Therefore, the method of manufacturing process of thesymmetrical TVS circuit includes a step of configuring the first andsecond MOSFET transistors as two lateral MOSFET transistors sharing acommon source region and encompassed the first and second MOSFETtransistors in a P-body region to function as the base of the NPNtransistor. The method further includes a step of supporting the P-bodyregion with a bottom N-doped region for combing with the P-body regionfunctioning as a base interposed between the common source regionfunctioning as an anode and the bottom doped region functioning as acathode for the NPN transistor formed as a vertical NPN transistor inthe semiconductor substrate. Specifically, the process may start withforming a N Epi layer 155′ on top of a N+ substrate 150′ followed byimplanting a P-well 140-B in the Epi layer. The source regions 110-S and120-S and drain regions 110-D and 120-D are then implanted using a mark.Gate 110-G and 120-G may be formed before or after the source/drainimplant by blanket deposition of a poly layer over a thermally formedgate oxide then etch off with a mask. A dielectric layer may be formedon the top surface followed by contact opening and P+ body contact 125implant. In a preferred embodiment, the process includes a step ofapplying a double metal layer manufacturing process to electricallyconnect anode and cathode to corresponding semiconductor regions. Theanode 105′ on the top surface of semiconductor and the anode 105 on thebottom surface of semiconductor may be electrically connected on aleadframe (not shown) of a package or on the chip, such as forming a N+sinker (not shown) from top down to the N+ substrate.

Referring to FIGS. 6A to 6C for a series of perspective view to showstandard CMOS processing steps to manufacture a TVS device as that shownin FIG. 4. In FIG. 6A, a P+ substrate 205 supporting a P-doped epitaxiallayer 210 is implanted to form two N-well 215. In FIG. 6B, a gate oxidelayer 225 is first formed followed by deposition of a polysilicon gatelayer and patterned into two gate segments 220-1 and 220-2 designated asM1 gate and M2 gate. The gate segments are each padded by the gate oxidelayer 225-1 and 225-2 respectively. Then, in FIG. 6C, a mask (not shown)is applied to carry out a source and drain implant to form the sourceand drain regions 230-S and 230-D for transistor M1 and 240-S and 240-Dfor transistor M2. With the interconnecting source regions 230-S and240-S for as a N+ source region. The manufacturing processes continuewith the formation of an oxide/BPSG layer and opening a P body contactopening. A body contact implant is performed to dope a P+ body contact250. Then deposition and patterning of two metal layers (not shown) areperformed to complete the device manufacturing processes of a TVS deviceshown in FIG. 4.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

1. A method of manufacturing a symmetrical blocking transient voltagesuppressing (TVS) circuit comprising: implanting a first and secondwells of a first conductivity type in an epitaxial layer of a secondconductivity type constituting a third well of the second conductivitytype between the first and second wells supported on a substrate of thesecond conductivity type; forming a first and second gates padded with agate insulation layer on a top of the third well laterally opposite fromthe first and second doped wells; and applying an implant mask to carryout a source and drain implant with dopant of the first conductivitytype for forming a source region and a drain region on two oppositesides of the first and second gates to form two transistors withinterconnecting source regions.
 2. The method of claim 1 furthercomprising: removing the implant mask followed by covering a top surfacewith an insulation layer and opening a body contact open; and carryingout a body contact implant through the body contact opening followed bydepositing and patterning a metal layer to complete interconnections ofthe symmetrical blocking TVS circuit.
 3. The method of claim 1 wherein:the step of implanting the first and second wells further comprising astep of implanting a first and second N-wells in a P-type epitaxiallayer supported on a P-type substrate.
 4. The method of claim 2 wherein:the step of implanting the source and drain region further comprising astep of implanting an N type dopant to form the source and drain regionson the two opposite sides of the first and second gates, the sourceregion disposed along a side of the first gate extending across a gapbetween the first and second gates to a region along a side of thesecond gate.
 5. The method of claim 1 wherein: the step of forming thefirst and second transistors further comprising a step of forming thetwo transistors as two substantially identical transistors for carryingout a substantially symmetrical bi-directional clamping a transientvoltage.
 6. The method of claim 1 wherein: the step of forming the firstand second transistors further comprising a step of forming said twotransistors as a first and a second MOSFET transistors having with saidinterconnecting source regions.
 7. The method of claim 4 wherein: saidstep of forming said source and drain regions by implanting the N-typedopant in the third well of the P type laterally opposite from the firstand second wells of the N-type further comprising a step of forming aNPN bipolar transistor across the first and second wells and the P-typeepitaxial layer.
 8. The method of claim 7 wherein: the step of formingthe NPN transistor further comprising forming said first and secondwells of the N-type as an emitter and a collector respectively and saidthird well of the P-type as a base for said bipolar transistor.
 9. Themethod of claim 1 wherein: said step of forming said first and secondtransistors further comprising a step of forming the first and secondtransistors as two lateral MOSFET transistors with interconnectingsource regions and forming said first and second MOSFET transistors inan area on top of the third well of a P-type conductivity typefunctioning as a base of a NPN transistor.
 10. A method of manufacturinga symmetrical blocking transient voltage suppressing (TVS) circuitcomprising: forming an epitaxial layer of a first conductivity typesupported on a substrate of the first conductivity type; implanting adoped well of a second conductivity type opposite the first conductivitytype in the epitaxial layer; forming a first and second gates paddedwith a gate insulation layer on top of the doped well of secondconductivity type; applying an implant mask to carry out a source anddrain implant with dopant of the first conductivity type for forming asource region and a drain region inside the doped well of the secondconductivity type to form two MOSFET transistors; and electricallyconnecting the first gate to the drain region disposed along a side ofthe second gate and the second gate to the drain region disposed along aside of the first gate respectively.
 11. The method of claim 10 furthercomprising: removing the implant mask followed by covering a top surfacewith an insulation layer and opening a body contact open; and carryingout a body contact implant through the body contact opening followed bydepositing and patterning a metal layer to complete interconnections ofthe symmetrical blocking TVS circuit.
 12. The method of claim 10wherein: the step of implanting the doped well of second conductivitytype further comprising a step of implanting a P-type dopant into anN-type epitaxial layer supported on an N-type substrate.
 13. The methodof claim 11 wherein: the step of implanting the source and drain regionfurther comprising a step of implanting an N type dopant to form thesource and drain regions on two opposite sides of the first and secondgates.
 14. The method of claim 10 wherein: the step of forming the firstand second MOSFET transistors further comprising a step of forming thetwo MOSFET transistors as two substantially identical MOSFET transistorsfor carrying out a substantially symmetrical bi-directional clamping atransient voltage.
 15. The method of claim 10 wherein: the step offorming the first and second MOSFET transistors further comprising astep of forming said two MOSFET transistors as a first and a secondMOSFET transistors having interconnecting source regions.
 16. The methodof claim 12 wherein: said step of forming said source and drain regionsby implanting the N-type dopant in the doped well of the P type furthercomprising a step of forming a NPN bipolar transistor across the drainregions, the P-type doped well and the N-type substrate.
 17. The methodof claim 10 wherein: the step of forming the said source and drainregions by implanting the N-type dopant in the doped well of the P typebeing carried out before the step of forming the first and second gates.18. The method of claim 10 wherein: said step of forming said first andsecond MOSFET transistors further comprising a step of forming the firstand second MOSFET transistors as two lateral MOSFET transistors withinterconnecting source regions and forming said first and second MOSFETtransistors in an area on top of the doped well of the secondconductivity type functioning as a base of a bipolar transistor.